Gyratory crushers are used for crushing ore, mineral and rock material to smaller sizes. Typically, the crusher comprises a crushing head mounted upon an elongate main shaft. A first crushing shell (typically referred to as a mantle) is mounted on the crushing head and a second crushing shell (typically referred to as a concave) is mounted on a frame such that the first and second crushing shells define together a crushing chamber through which the material to be crushed is passed. A driving device positioned at a lower region of the main shaft is configured to rotate an eccentric assembly positioned about the shaft to cause the crushing head to perform a gyratory pendulum movement and crush the material introduced in the crushing chamber. Example gyratory crushers are described in WO 2004/110626; WO 2008/140375, WO 2010/123431 and WO 2012/005651.
Primary crushers are heavy-duty machines designed to process large material sizes of the order of one meter. Secondary and tertiary crushers are however intended to process relatively smaller feed materials typically of a size less than fifty centimetres. Cone crushers represent a sub-category of gyratory crushers and may be utilised as downstream for final processing of materials. However, common to all types of gyratory crushers is a requirement to crush the material according to a predetermined reduction an as to obtain a desired particulate size of material exiting the crusher. WO 2006/101432 discloses an inner crushing shell having a series of raised crushing surfaces that project radially from the outward facing surface of the shell wall that are configured to provide a variable gap distance between the outer crushing shell to accommodate and crush a range of different sized pieces of material within the crushing zone.
One of the most common user demands on gyratory crushers is high reduction. Reduction is however restricted by limitations of energy consumption (power draw) and hydraulic pressure which are both related to the crushing force. Crushing dynamics principally involve the material pieces being trapped, compressed and then crushed in the zone between the mantle and the concave as they fall through the crusher. The crushing process is complex and the performance of the crusher is determined by a number of factors including i) the size distribution of material as it enters the crusher ii) the dynamics of the material as it is crushed and breaks; iii) the machine operating parameters including for example the close side setting (CSS), open side setting (OSS), stroke and speed and iv) the geometry of the machine and the crushing zone including in particular the gap between the concave and the mantle in to which the material falls.
One problem with existing crushers is the undesirable frequency with which the crusher ‘chokes’. This occurs as the crusher allows entry of more material than what can be crushed in the lower crushing zones (below the choke point) due to limitation in the available crushing force. A result of this choking is that the force is insufficient to crush the material in the gap and the crusher can no longer retain the CSS. The crusher must then open, typically an automated process, to allow the choked material to exit the crusher and the crusher effectively reset. What is required is a gyratory crusher that addresses these problems and the disruption caused by choking.